Add Char.to_i128 and .to_u128

[straight-shoota]

Resolves #12892

Related to #12884

Also deprecates Enum.flags which is replaced by Enum.[].

[asterite]

What's the point of doing this with a non-flags enum? Maybe the old method should have raised a compile error in that case. And now I'm thinking flags is a better name than brackets.

[straight-shoota]

Yeah, that's a legitimate question.Enum.flags already works for non-flags enums as well. And I think that's fine. Also for Enum.[].
It's definitely very useful for single arguments to provide a valid-code syntax for Enum#inspect that's unique and concise (see examples #12884).
Enum#| is also defined for non-flag enums, so even multiple arguments are valid. Not sure if there's a good use case for that, but as long as | works, there's no point in restricting the constructor IMO (ofc we could do that and allow only a single argument for non-flag enums, but I don't see much point in that).

[beta-ziliani]

I'm worried about the amount of deprecation messages that we will throw in the face of the entire Crystal ecosystem's devs. I wonder if we shouldn't allow ourselves to have two ways of doing it for a while, mention in the docs that .flags is deprecated, and wait for a while until doing the actual deprecation.

[asterite]

What's wrong with keeping the name flags?

[straight-shoota]

Sure, let's keep Enum.flags for now.

@asterite It's effectively an alias for Enum.[] now, which is preferable for its conciseness.